Biodiesel is a natural and renewable domestic fuel alternative for diesel engines made from vegetable oils and fats. Because it is nontoxic and biodegradable it has become a promising alternative to fuels made from petroleum. Biodiesel burns clean. Thus, it results in a significant reduction of the types of pollutants that contribute to smog and global warming. Biodiesel emits up to 85 percent fewer cancer-causing agents and is the only alternate fuel approved by the Environmental Protection Agency (EPA). It has passed every Heath-Effects Test of the Clean Air Act and meets the requirements of the California Air Resources Board (CARB). Although, biodiesel is still relatively costly to make, the utilization of its co-product glycerol is one of the promising options for off-setting the biodiesel production cost.
Glycerol has more than 1500 known applications in many different industries ranging from foods, pharmaceuticals, and cosmetics (i.e., USP grade glycerol) to paints, coatings and other industrial types of uses (i.e., technical grade glycerol). It is the most versatile and valuable by-product created during biodiesel production. One gallon of biodiesel generates about 1.05 pounds of crude glycerol. A 30-million-gallon-per-year plant generates about 11,500 tons of 99.9 percent pure glycerol. It is speculated that the world market will generate approx. 37 billion gallons of biodiesel by 2016, suggesting a production of 4 billion gallons or 16.5 million metric tons of crude glycerol. This is believed to create too much of a crude glycerol surplus which may negatively impact the refined glycerol market (Yang et al. (2012) Biotechnology for Biofuels 5:13). According to the EPA, this impure form of glycerol must be disposed of within a certain period of time, leading to high disposal fees for companies that produce glycerol as a by-product. Hence, the development of sustainable methods for utilizing raw organic glycerol is desirable while it is equally desirable not to offset the balance of crude to refined product.
Most biodiesel productions use homogeneous alkaline catalysts such as sodium methylate. The transesterification of triacylglycerides with methanol creates a methyl-ester phase and a glycerol phase. Impurities, including catalyst, soap, methanol and water are usually concentrated in the glycerol phase. The glycerol phase is generally neutralized with acid and the cationic component of the catalyst is incorporated as a salt. Thus, it is not uncommon that glycerol, as a by-product of the biodiesel production, has a salt content of 5 to 7 percent. This high salt content makes conventional purification techniques cost intensive. There are various methods for purifying crude glycerol, including fractional distillation, membrane technology employing a series of NF and RO membrane stages (NF/RO membrane), electro-dialysis membrane technology (electro-dialysis membrane), bipolar membrane technology (bi-polar membrane), and ion-exchange resin adsorption technology (ion-exchange resin adsorption). Fractional distillation is the most commonly practiced method. It results in high purity glycerol at high yields, however, it is also capital-, labor-, and energy-intensive. Glycerol has a high heat capacity and, thus, requires a high-energy input for vaporization. Another common technique for glycerol purification is the classical ion-exchange method. But the higher salt content of glycerol as a result of biodiesel production makes classical ion-exchange an uneconomical choice. Particularly, the chemical regeneration cost for the resins becomes too high when the salt content in glycerol approaches 5 to 7 percent.
Most methods that are used to purify glycerol are based on aqueous technologies that use crude glycerol water, i.e., they use glycerol that contains about 60 to 70 percent water as a feed. Fractional distillation refines glycerol by using crude glycerol that contains about 6 to 8 percent water that has gone through methanol rectification and water evaporation. Amongst all the available technologies, the electro-dialysis membrane, bi-polar membrane and ion-exchange resin adsorption are mainly desalting processes. They all require separate deoiling (i.e., de-oiling) process steps and generate large amounts of waste water. The ion-exchange resin adsorption method is mainly used for low salt polishing applications. The NF/RO membrane uses a multi-stage membrane unit for the glycerol refining process that is capable of both desalting and deoiling the glycerol.
There are also hybrid systems for purifying crude glycerol. For example, a hybrid system for purifying glycerol can employ a membrane technology as a main process and distillation as a minor process, wherein both can recover glycerol in so-called concentrate and permeate streams. In that type of system, the concentrate stream contains dirty glycerol water while a permeate stream contains cleaner glycerol water. The glycerol contained within the concentrate streams can be recovered or discharged as a loss. Each stage that contains a permeate stream in a process that applies any of the membrane technologies (i.e., a membrane process) contains glycerol-water intermediates with reduced salt and reduced organics. Each stage that contains a concentrate stream in a membrane process contains glycerol, water, concentrated salt and concentrated organic impurities. Fractional distillation can also be used in a hybrid system. Fractional distillation is similar to a membrane system in that it is capable of desalting and deoiling the glycerol but it relies on continuous salt removal under high vacuum. A hybrid system employing fractional distillation recovers glycerol in the concentrate stream of the membrane process.
Although, both fractional distillation and NF/RO membrane produce glycerol suitable for fermentation, the high production cost creates a down side. Currently, the majority of large industrial commercial processes employ fractional distillation. The equipment cost of fractional distillation of crude glycerol is high due to the need of continuous salt removal under high vacuum (see, e.g., Glycerine a Key Cosmetic Ingredient, Cosmetic Science and Technology Series (1991) by Marcel Dekker, Inc; and Bailey's Industrial Oil and Fat Products, Sixth Edition, Six Volume Set (2005) by John Wiley and Sons, Inc.).
Purified or refined glycerol (i.e., USP grade glycerol) has numerous applications from fragrances to cosmetics to pharmaceuticals and is a valued commercial product. The production of purified glycerol is costly because the majority of existing methods of purification employ fractional distillation (supra). However, USP grade glycerol is not suitable for all applications because it is simply too costly to manufacture and unnecessarily pure for industrial applications (e.g., paints, coats, adhesives, etc.). Technical grade glycerol is more suitable for industrial applications but its production also relies on fractional distillation and it is therefore not a cost-effective alternative. Thus, a method is needed that produces a form of technical grade glycerol at a low enough cost that is acceptable for industrial applications. In addition, there is a need for a new form of technical grade glycerol with characteristics that meet the specifications required for renewable methods and bio-degradable products that reach beyond those that rely mostly on refined or crude glycerol. The present disclosure addresses this need.